In an x-ray photoelectron spectroscopic (XPS) instrument, a beam of x-rays illuminates a portion of a specimen to cause electrons to be emitted, for example as disclosed in U.S. Pat. No. 5,315,113 (Larson et al.). These emissions are analyzed with an energy analyzer to determine the composition of the surface. However, with an insulating specimen, the electron emissions leave the surface positively charged in the region of the x-ray illumination. The positive charge varies across the surface, thereby affecting the emitted electron energies and trajectories, and introducing errors into corresponding analyses.
In secondary ion mass spectroscopy (SIMS) a surface is irradiated with positive ions so as to cause emission of atoms and ions from the surface. The incident ions cause a buildup of positive charge on an insulating specimen. As for XPS, such a charge introduces errors into analyses.
Various approaches to these problems include, at least for XPS, interposing a grid close to the sample to smooth gradients in electrical potential, such as disclosed in U.S. Pat. No. 4,680,467 (Bryson). This introduces an interfering element and has limited applicability.
In another approach, the specimen is neutralized by flooding with low energy electrons. This provides a significant improvement, but generally results in non-uniform neutralization because an area larger than the region of photoemission is flooded, thereby supplying excess negative charge in the area outside of the photoelectron region. U.S. Pat. No. 5,432,345 ("Kelly patent", assigned to the present assignee) discloses smoothing the field gradients by discharging excess negative charge with irradiation of ultraviolet light or a beam of positive ions. This technique with radiation or ions can reduce the gradients quite significantly, but there remains some gradient with resulting effects on analyses. Also, the Kelly reference is directed primarily to ultraviolet radiation and gives few details in the use of positive ions. Ion sources are normally used for sputtering and the like, and typically produce ions with energies greater than 10 eV, although lower energy devices have been used experimentally.
A general problem with existing XPS instruments, even utilizing electron flooding, is sensitivity to neutralizer operating conditions. It has been difficult to obtain reproducible photoelectron peak positions and peak shapes.